There has been substantial interest in exploiting the properties of compound semiconductors consisting of particles with dimensions in the order of 2-50 nm, often referred to as quantum dots (QDs) or nanocrystals. These materials are of commercial interest due to their size-tuneable electronic properties which can be exploited in many commercial applications such as optical and electronic devices, and other applications that range from biological labelling, photovoltaics, catalysis, biological imaging, LEDs, general space lighting and electroluminescent displays, amongst many new and emerging applications.
The most studied of semiconductor materials have been the chalcogenide II-VI materials, namely ZnS, ZnSe, CdS, CdSe, CdTe; most noticeably CdSe due to its tuneability over the visible region of the spectrum. Reproducible methods for the large scale production of these materials have been developed from “bottom up” techniques, whereby particles are prepared atom-by-atom, i.e., from molecules to clusters to particles, using “wet” chemical procedures.
Two fundamental factors, both related to the size of the individual semiconductor nanoparticle, are responsible for their unique properties. The first is the large surface to volume ratio; as a particle becomes smaller, the ratio of the number of surface atoms to those in the interior increases. This leads to the surface properties playing an important role in the overall properties of the material. The second factor being, with many materials including semiconductor nanoparticles, that there is a change in the electronic properties of the material with size; moreover, because of quantum confinement effects the band gap gradually becomes larger as the size of the particle decreases. This effect is a consequence of the confinement of an ‘electron in a box’ giving rise to discrete energy levels similar to those observed in atoms and molecules, rather than a continuous band as observed in the corresponding bulk semiconductor material. Thus, for a semiconductor nanoparticle, because of the physical parameters, the electron and hole produced by the absorption of electromagnetic radiation (a photon, with energy greater than the first excitonic transition), are closer together than they would be in the corresponding macrocrystalline material, moreover the Coulombic interaction cannot be neglected. This leads to a narrow bandwidth emission, which is dependent upon the particle size and composition of the nanoparticle material. Thus, quantum dots have higher kinetic energy than the corresponding macrocrystalline material and consequently the first excitonic transition (band gap) increases in energy with decreasing particle diameter.
Core semiconductor nanoparticles, which consist of a single semiconductor material along with an outer organic passivating layer, tend to have relatively low quantum efficiencies due to electron-hole recombination occurring at defects and dangling bonds situated on the nanoparticle surface which can lead to non-radiative electron-hole recombinations. One method to eliminate defects and dangling bonds on the inorganic surface of the quantum dot is to grow a second inorganic material, having a wider band-gap and small lattice mismatch to that of the core material, epitaxially on the surface of the core particle to produce a “core-shell” particle. Core-shell particles separate any carriers confined in the core from surface states that would otherwise act as non-radiative recombination centres. One example is a ZnS shell grown on the surface of a CdSe core. Another approach is to prepare a core-multi shell structure where the electron-hole pair is completely confined to a single shell layer consisting of a few monolayers of a specific material such as a quantum dot-quantum well structure. Here, the core is of a wide band gap material, followed by a thin shell of narrower band gap material, and capped with a further wide band gap layer, such as CdS/HgS/CdS grown using substitution of Hg for Cd on the surface of the core nanocrystal to deposit just a few monolayers of HgS, which is then over-grown by a monolayer of CdS. The resulting structures exhibit clear confinement of photo-excited carriers in the HgS layer. To add further stability to quantum dots and help to confine the electron-hole pair one of the most common approaches is by epitaxially growing a compositionally graded alloy layer on the core; this can help to alleviate strain that could otherwise led to defects. Moreover, for a CdSe core, in order to improve structural stability and quantum yield, rather growing a shell of ZnS directly on the core a graded alloy layer of Cd1-xZnxSe1-ySy can be used. This has been found to greatly enhance the photoluminescence emission of the quantum dots.
Doping quantum dots with atomic impurities is an efficient way also of manipulating the emission and absorption properties of the nanoparticle. Procedures for doping of wide band gap materials, such as zinc selenide and zinc sulfide, with manganese and copper (ZnSe:Mn or ZnS:Cu), have been developed. Doping with different luminescence activators in a semiconducting nanocrystal can tune the photoluminescence and electroluminescence at energies even lower than the band gap of the bulk material, whereas the quantum size effect can tune the excitation energy with the size of the quantum dots without having a significant change in the energy of the activator-related emission.
The widespread exploitation of quantum dot nanoparticles has been restricted by their physical/chemical instability and incompatibility with many of the materials and/or processes required to exploit the quantum dots to their full potential, such as incorporation into solvents, inks, polymers, glasses, metals, electronic materials, electronic devices, bio-molecules and cells. Consequently, a series of quantum dot surface modification procedures has been employed to render the quantum dots more stable and compatible with the materials and/or processing requirements of a desired application.
A particularly attractive potential field of application for quantum dots is the development of next generation light-emitting diodes (LEDs). LEDs are becoming increasingly important in modern-day life and it is envisaged that they have the potential to become one of the major applications for quantum dots, in for example, automobile lighting, traffic signals, general lighting, liquid crystal display (LCD) backlighting and display screens.
Currently, LED devices are made from inorganic solid-state compound semiconductors, such as AlGaAs (red), AlGaInP (orange-yellow-green), and AlGaInN (green-blue); however, using a mixture of the available solid-state compound semiconductors, solid-state LEDs which emit white light cannot be produced. Moreover, it is difficult to produce “pure” colours by mixing solid-state LEDs of different frequencies. Therefore, currently the main method of colour mixing to produce a required colour, including white, is to use a combination of phosphorescent materials which are placed on top of the solid-state LED whereby the light from the LED (the “primary light”) is absorbed by the phosphorescent material and then re-emitted at a different frequency (the “secondary light”), i.e., the phosphorescent materials down-convert the primary light to the secondary light. Moreover, the use of white LEDs produced by phosphor down-conversion leads to lower cost and simpler device fabrication than a combination of solid-state red-green-blue LEDs.
Current phosphorescent materials used in down-converting applications absorb UV or mainly blue light and convert it to longer wavelengths, with most phosphors currently using trivalent rare-earth doped oxides or halophosphates. White emission can be obtained by blending phosphors which emit in the blue, green and red regions with that of a blue or UV-emitting solid-state device, i.e., a blue-light-emitting LED plus a green phosphor such as, SrGa2S4:Eu2+, and a red phosphor such as, SrSiEu2+ or a UV-light-emitting LED plus a yellow phosphor such as, Sr2P2O7:Eu2+;Mu2+, and a blue-green phosphor. White LEDs can also be made by combining a blue LED with a yellow phosphor; however, colour control and colour rendering are poor when using this methodology due to lack of tunability of the LEDs and the phosphor. Moreover, conventional LED phosphor technology uses down-converting materials that have poor colour rendering (i.e., colour rendering index (CRI)<75).
Rudimentary quantum dot-based light-emitting devices have been made by embedding colloidally produced quantum dots in an optically clear (or sufficiently transparent) LED encapsulation medium, typically a silicone or an acrylate, which is then placed on top of a solid-state LED. The use of quantum dots potentially has some significant advantages over the use of the more conventional phosphors, such as the ability to tune the emission wavelength, strong absorption properties and low scattering if the quantum dots are mono-dispersed.
For the commercial application of quantum dots in next-generation light-emitting devices, the quantum dots should be incorporated into the LED encapsulating material while remaining as fully mono-dispersed as possible and without significant loss of quantum efficiency. The methods developed to date are problematic, not least because of the nature of current LED encapsulants. Quantum dots can agglomerate when formulated into current LED encapsulants, thereby reducing the optical performance of the quantum dots. Moreover, even after the quantum dots have been incorporated into the LED encapsulant, oxygen can still migrate through the encapsulant to the surfaces of the quantum dots, which can lead to photo-oxidation and, as a result, a drop in quantum yield (QY).
In view of the significant potential for the application of quantum dots across such a wide range of applications, including but not limited to, quantum dot-based light-emitting devices, work has already been undertaken by various groups to try to develop methods to increase the stability of quantum dots so as to make them brighter, more long-lived, and/or less sensitive to various types of processing conditions. For example, reasonably efficient quantum dot-based light-emitting devices can be fabricated under laboratory conditions building on current published methods; however, there remain significant challenges to the development of quantum dot-based materials and methods for fabricating quantum dot-based devices, such as light-emitting devices, on an economically viable scale and which would provide sufficiently high levels of performance to satisfy consumer demand.